1. Field of the Invention
The present invention relates to an information storage medium such as an optical disk formed with a wobbled track. The present invention also relates to an information storage medium evaluation method and information storage medium evaluation apparatus, which evaluate the quality of such information storage medium. The present invention relates to an information reproduction apparatus and information reproduction method, which reproduce information from such information storage medium. Furthermore, the present invention relates to an information recording apparatus for recording information on such information storage medium.
2. Description of the Related Art
As is well known, in recent years, optical disks having a single-layer/single-sided size of 4.7 GB are commercially available as those which can achieve high-density recording of information. For example, a DVD-ROM as a read-only optical disk, and a rewritable DVD+RW (ECMA-337), DVD-RW (ECMA-338), and DVD-RAM (ECMA-330) are available.
An information recording layer is formed on a transparent substrate on each of these optical disks. An information recording layer of such optical disk has a guide groove called a “groove”. Recording/reproduction of information on/from an optical disk is made along this guide groove. By focusing a laser beam on the guide groove of the information recording layer, information is recorded on this guide groove or information recorded on that guide groove is reproduced.
For example, physical addresses used to specify a spatial position where information is to be recorded/reproduced are recorded on the DVD-RAM. For example, these physical addresses are formed to obstruct the guide groove.
By contrast, on a +RW disk, physical addresses are reflected on the guide groove using groove wobble modulation (to be referred to as wobble modulation hereinafter) that radially slightly wobbles the guide groove. This method changes the wobble phase in correspondence with information to be recorded (physical address), and its technique is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 10-283738. Physical addresses which are recorded by such wobble modulation do not obstruct the recording track. That is, the physical addresses recorded by wobble modulation do not limit the recording area of user information on a disk. Hence, wobble modulation can assure high format efficiency, and can easily achieve compatibility to read-only media.
As an evaluation measure of the quality of a wobble signal formed by optically reproducing the groove wobbles, the Narrow Band Signal to Noise Ratio (NBSNR) of the wobble signal is known. This value is used to evaluate the ratio of the amplitude of a carrier that carries the wobble signal to that of noise, and indicates a higher demodulation ratio with increasing NBSNR. This NBSNR is also called a Carrier to Noise Ratio (CNR).
Normally, the NBSNR of a wobble signal is measured by inputting a wobble signal to a frequency component analysis device such as a spectrum analyzer or the like, and calculating the difference between the peak value of a carrier frequency and the noise level near the carrier frequency. However, when the wobble signal contains modulated components, the peak value of the carrier frequency becomes lower than the actual one. Also, the frequency of the modulated components raises the signal level near the carrier frequency. Hence, when the wobble signal contains modulated components, the NBSNR of the wobble signal cannot be accurately measured.
On the other hand, a wobble signal obtained from a +RW disk contains two different components, i.e., non-modulated and modulated components, and most of components are non-modulated components. For this reason, the NBSNR of the wobble signal can be measured by practically disregarding modulated components. However, when a modulated region is reduced, an information size that can be recorded becomes smaller. Hence, when a recording capacity of information by wobble modulation is increased, this method cannot be used.
In order to maintain a high-quality wobbled track, the NBSNR of a wobble signal must be accurately measured. If the NBSNR of the wobble signal cannot be accurately measured, the quality of a wobble track may deteriorate. If a wobble track has poor quality, physical address information reflected on the wobble track cannot be normally reproduced. Hence, correct information may not consequently reproduced from a disk. Likewise, correct information may not be recorded on a disk.